1. Field of the Invention
The present invention relates to an image forming apparatus and a droplet ejection correction method, and more particularly, to an image forming apparatus for correcting droplet ejection in cases where images are formed by droplet ejection and a droplet ejection correction method.
2. Description of the Related Art
Among image forming apparatuses which form images by ejecting and depositing droplets of ink onto a medium, such as paper, apparatuses are known which read in an image formed on a medium and correct ink droplet ejection on the basis of the reading results (see, for example, Japanese Patent Application Publication Nos. 5-238012, 6-166247, 6-198866 and 7-266582).
Furthermore, an image forming apparatus is also known which reads in an image formed on a medium and determines ejection failure nozzles on the basis of the reading results (see, for example, Japanese Patent Application Publication No. 6-297728).
In a so-called single-pass image forming apparatus which forms images by moving a medium, such as paper, once only, with respect to a head having a plurality of nozzles, any variation in the droplet ejection between respective nozzles, such as variation in the direction of flight of the ejected droplets and variation in the ejected droplet volumes, directly produces image abnormalities, such as so-called “banding”, or the like.
In an image forming apparatus of this kind, naturally, better reliability of the head is achieved, but even in this case, if using a head having a very large number of nozzles, it is not possible to avoid droplet ejection variations which occur with a certain probability.
If it is sought to correct droplet ejection on the basis of image reading results, in order to avoid image deterioration due to variations in droplet ejection, then “instable variations” present a major problem. More specifically, if the variation in the droplet ejection is “relatively stable variation” which involves gradual variation over time, then if images containing relatively stable variation of this kind are read in and correction is performed directly on this basis, then an effect in maintaining high image quality is obtained; but if the variation in droplet ejection is “instable variation” which involves sudden variation over time, then even if images containing “instable variation” of this kind are read in and correction is performed directly on this basis, when the correction is actually implemented, it is possible that the direction of variation will have changed (in other words, the direction of flight of the ejected droplets or the direction of increase or decrease in the ejection droplet volume will have changed), and in cases such as this, the deterioration of the image may actually be exacerbated. In other words, there is a problem in that high-frequency noise components adversely affect the correction.
Well-known and commonly used methods for removing high-frequency noise components are simple averaging, such as arithmetic averaging, and filtering of the high-frequency components, but in the case of an image forming apparatus, increasing the number of measurement operations so as to achieve effective noise removal means increasing the number of prints or increasing the number of test patterns. Therefore, it is not possible to increase the number of measurement operations, readily.
Japanese Patent Application Publication Nos. 5-238012, 6-166247, 6-198866, 7-266582, and 6-297728 mentioned above do not make any concrete description of the problem of dealing with temporal variation in droplet ejection, and nor do they present ways of resolving this problem.
Japanese Patent Application Publication No. 6-198866 discloses a general averaging process between pixels, namely, a process which uses the average value of the adjacent pixels to each pixel (dot) output, (for example, the average of three pixels), as the output for that pixel, in order to reduce the effects of noise and reading error, but it does not discuss how to deal with temporal variations in droplet ejection.